Method and means for measuring operating temperature of semiconductor devices by monitoring RF characteristics
BACKGROUND OF THE INVENTION 
     This invention relates generally to semiconductor integrated circuit test apparatus, and more particularly the invention relates to a method and apparatus for measuring operating temperature of the active area of a semiconductor device. 
     The operating temperature of semiconductor devices, particularly gallium arsenide FETs employed in microwave circuits, is an important parameter in military and space applications. The operating temperature of the active area directly relates to the operating lifetime of the device. 
     However, semiconductor die are too small to use conventional contact probing techniques to measure their temperature. Three alternative techniques are presently used and are the basis for several commercially available products. 
     In one technique, a high-magnification microscope is used in conjunction with an infrared detector. The intensity of the infrared radiation from the focused area is a strong function of temperature and can, in principle, be detected and measured. However, different materials have widely varying emissivities which makes the accuracy of any measurement critically dependent on an in situ calibration procedure and on the precise positioning of the sample area. Accuracy and repeatability are difficult to achieve in practical use; however, high-volume manufacturing and screening applications have used the technique because of the nonintrusive nature thereof. 
     The optical properties of liquid crystals can be employed in monitoring temperature. However, this is essentially a destructive technique which can only be used in sample tests. The test device has to be coated with the crystals and cannot be reused in its normal application. The technique does have the advantage of very high resolution and is typically used to locate hot spots. Only one temperature can be measured per crystal type. 
     The electrical characteristics of the device itself can be used as a temperature-sensitive parameter. Conventionally, pulsed DC characteristics are monitored such as the forward voltage-current characteristic of the base-emitter junction of a bipolar transistor. For example, Sage Enterprises Inc. of Mountain View, Calif. manufactures thermal resistance testers which monitor the forward voltage drop of a diode junction, such as the emitter-base junction of a transistor. The selected parameter under steady-state operating conditions is compared with the same parameter shortly after power is applied and before the device has had time to heat significantly. The apparatus advantageously employs the active region being measured as the sensing element. However, in practical use, the apparatus is limited mainly to discrete, single devices in packages that give access to all of the device terminals. 
     The present invention is directed to an improved apparatus and technique for measurement of the operating temperature of the active region in a semiconductor device where terminals of an active component therein are not readily accessible. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is an improved method of measuring the operating temperature of an active region of a semiconductor device. 
     Another object of the invention is test apparatus for readily implementing the measuring of operating temperature in accordance with the invention. 
     Still another object of the invention is greater accuracy, resolution and repeatability in the measuring of device operating temperature. 
     A feature of the invention is the use of RF characteristics of a device to sense temperature of the device. 
     Briefly, in accordance with one embodiment of the invention, the temperature of the active region of a gallium arsenide field-effect transistor used in a microwave amplifier circuit is determined. A gallium arsenide field-effect transistor has a well-defined and monotonic decrease of small signal gain versus increasing temperature. RF gain of the microwave amplifier is measured on a transient basis as the device is heated up from an ambient temperature. 
     Apparatus in accordance with the invention includes means for holding a device undergoing test, signal generator means for applying an input signal to the device undergoing test, and detector means for detecting an output signal from the device undergoing test in response to the input signal. Control means is provided for periodically supplying bias voltages to the device undergoing test, and sample-and-hold means responsive to the control means samples the output from the detector means at different times while a bias voltage is applied to the device undergoing test. Means then measures the sampled output for different periods after a bias voltage is applied and provides a measure of operating temperature therefrom. 
     Advantageously, a measurement window of a few microseconds is used to measure the RF gain of the device undergoing test with the position of the measurement window being moved from the instant bias is applied to the device undergoing test to several milliseconds after applying a bias voltage by appropriate setting of the control voltage. A typical thermal time constant may be on the order of one millisecond; therefore by observing the change in gain as the measurement window is swept from zero to 5 milliseconds, the heating of the active device area can be directly observed. 
     The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a functional block diagram of a temperature measurement apparatus in accordance with the invention. 
     FIG. 2 is a functional block diagram of sample-and-hold circuitry in the apparatus of FIG. 1. 
     FIG. 3 is a functional block diagram of the control unit in the apparatus of FIG. 1. 
     FIG. 4 is a timing diagram in the operation of the apparatus of FIG. 1. 
     FIG. 5 is a plot illustrating the operating temperature characteristics of devices using the apparatus of FIG. 1. 
     FIG. 6 is a plot illustrating the use of the apparatus of FIG. 1 with a fixture for heating the device under test and obtaining operating temperature characteristics at elevated temperatures. 
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Referring now to the drawing, FIG. 1 is a functional block diagram of apparatus in accordance with the invention. A device under test, such as a gallium arsenide field-effect transistor in a microwave amplifier circuit, for example, is mounted in a test fixture 10 which can be a conventional fixture as used in measuring RF characteristics of a microwave integrated circuit subassembly for a module. The signal generator 12 generates an input signal for application to the module in fixture 10, and the signal generator also provides a sweep voltage for a control unit 14. Control unit 14 responds to the sweep voltage from generator 12 and generates a constant duty cycle, variable width pulse, which is used to supply the bias voltage to the device under test. The pulse width can be varied over a wide, selectible range by varying the control voltage which is applied as an input to the control unit. 
     Detector 16 detects the output signal from the device under test, while bias voltage is applied to the device under test. A sample-and-hold unit 18 is connected to the detector and holds the detector output voltage in response to a command pulse provided by the control unit 14. With this arrangement, the RF gain of the device under test can be measured during a measurement window of a few microseconds. The position of the measurement window can be moved from the instant bias is first applied to the device under test to several milliseconds after application of the bias voltage by appropriate setting of the control voltage. A typical thermal time constant may be on the order of one millisecond; therefore by observing the change in gain as the measurement window is swept from zero to 5 milliseconds, the heating of the active device area can be directly observed. 
     The sample voltage from sample-and-hold unit 18 is applied to a video amplifier 20 which develops a plot of the thermal characteristics of the device under test. 
     FIG. 2 is a functional block diagram further illustrating the sample-and-hold unit 18 of FIG. 1. A conventional sample-and-hold chip 24 has a buffered input at 26 and a buffered output 28 to maintain the necessary interface impedance levels. A chopper 30 can be connected between the sample-and-hold chip 24 and the output buffer 28. The input buffer 26 requires a DC offset since the detector will usually operate with some DC bias that must be removed prior to chopping. 
     FIG. 3 is a more detailed block diagram of the control unit. A voltage-controlled clock oscillator 34 receives the control voltage from signal generator 12 and has a frequency output range of 1000:1. This clock output is divided down by 100 at 36 to obtain a constant duty cycle pulse train at point A. This divided output pulse train passes through logic at 38 to provide the sample command pulse at point B. Synchronized bias pulse is obtained from logic 40. The logic signal C is amplified and scaled at 42 to provide a variable voltage bias pulse to the device under test. The sample-and-hold command pulse is synchronized with the end of the bias pulse, but switches into hold mode before the bias pulse switches off. 
     FIG. 4 is an illustration of the timing signals at points A, B and C in the control unit block of FIG. 3. Signal A is one pulse of a constant duty cycle pulse train, signal B is the sample-and-hold pulse for the sample-and-hold command B, and pulse C is the bias pulse applied to the device under test. 
     Since gallium arsenide field-effect transistors have a well-defined gain-vs.-temperature characteristic which is fairly linear, a plot of the sampled output from the device under test swept through the time period of the application of the bias voltage gives a direct display of the heating of the active device area can be directly observed. A convenient way of recording the data is to use a ramp voltage to drive an x-y recorder and also provide the control voltage. A typical output plot is shown in FIG. 5. The vertical scale can be read directly in temperature if the gain conversion factor for the device under test is known. A more accurate absolute calibration can be performed if the test fixture is provided with a small heater and a temperature controller. The fixture can then be heated up until the gain at the zero time position corresponds to the desired measurement location on the thermal characteristics as illustrated in the plot of FIG. 6. 
     The operating temperature measuring apparatus in accordance with the invention provides greater accuracy, resolution, and repeatability than conventional test procedures. Further, the technique can be used to screen device temperature on complete assemblies since access to device terminals is not necessary. Useful comparative data can be obtained on assemblies containing a cascade of several devices. Such data would be adequate for screening out a poorly attached die, for example. 
     The apparatus is particularly useful in measuring relative and absolute operating temperature in the channel region of a gallium arsenide field-effect transistor mounted in a hybrid microwave integrated circuit. However, the technique and apparatus can be used with other types of semiconductor devices including silicon bipolar transistors, silicon MOS transistors, and gallium arsenide microwave integrated circuits (MMICs). Further, the apparatus can be used for measurement of RF and microwave amplifier under pulsed conditions and the characterization of RF and microwave transistors under pulsed conditions. 
     Thus, while the invention has been described with reference to a specific embodiment and application, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.